1. Technical Field
Example embodiments generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The image forming apparatuses are requested to form a high quality color toner image on a recording medium at high speed with downsized components. To address those requests, the fixing devices incorporated in the image forming apparatuses are requested to exert increased pressure to the toner image on the recording medium for an extended time while facilitating conveyance of the recording medium. Such fixing devices may employ various types of configurations such as a roller type and a belt type. FIG. 1 illustrates a roller type fixing device 25R1 including a fixing roller 91 and a pressing roller 92 pressed against the fixing roller 91 to form a fixing nip N therebetween through which a recording medium P bearing a toner image is conveyed. The fixing roller 91 is constructed of a metal tube 93 and a rubber layer 95 coating the metal tube 93 and accommodates a halogen heater 97. Similarly, the pressing roller 92 is constructed of a metal tube 94 and a rubber layer 96 coating the metal tube 94 and accommodates a halogen heater 98. As the recording medium P is conveyed through the fixing nip N, the fixing roller 91 heated by the halogen heater 97 and the pressing roller 92 heated by the halogen heater 98 apply heat and pressure to the recording medium P, fixing the toner image on the recording medium P. The rubber layers 95 and 96 may constitute an increased thermal resistance. To address this circumstance, the rubber layers 95 and 96 may be formed in a thin layer. However, the thin rubber layers 95 and 96 may produce the fixing nip N having a reduced length in a recording medium conveyance direction where the fixing roller 91 and the pressing roller 92 may apply heat and pressure to the recording medium P insufficiently.
FIG. 2 illustrates a belt type fixing device 25R2 including a fixing belt 77 stretched across a heating roller 78 and a fixing roller 74. A pressing roller 72 is pressed against the fixing roller 74 via the fixing belt 77 to form a fixing nip N between the pressing roller 72 and the fixing belt 77. As a recording medium P bearing a toner image is conveyed through the fixing nip N, the fixing belt 77 indirectly heated by a heater 84 through the heating roller 78 and the pressing roller 72 heated by a heater 86 apply heat and pressure to the recording medium P, thus fixing the toner image on the recording medium P.
Since the fixing roller 74 includes a rubber layer thicker than the rubber layer 95 of the fixing roller 91 of the fixing device 25R1 depicted in FIG. 1, the fixing roller 74 forms the greater fixing nip N. However, the fixing roller 74 is required to have a greater diameter that decreases the curvature of the fixing roller 74. Accordingly, the recording medium P, as it is discharged from the fixing nip N, may be wound around the fixing belt 77 stretched across the relatively great fixing roller 74. Further, the relatively great fixing roller 74 may upsize the fixing device 25R2.
In order to prevent the recording medium P from being wound around the fixing belt 77, the fixing belt 77 may be looped over a stationary, nip formation pad as disclosed by JP-2004-252354-A and JP-2004-198556-A. For example, a fixing belt may be looped over the nip formation pad and a smaller fixing roller to produce a fixing nip having an increased length in the recording medium conveyance direction. Although the fixing belt slides over the nip formation pad, the fixing roller decreases frictional resistance between the nip formation pad and the fixing belt sliding thereover, facilitating rotation of the fixing belt.
Since the nip formation pad and the fixing roller create an increased curvature of the fixing belt looped thereover, the recording medium discharged from the fixing nip separates from the fixing belt readily. However, the small nip formation pad may not endure increased pressure from the pressing roller.
In order to increase endurance against pressure from the pressing roller, a stationary, tubular heat conductor may be employed. For example, as disclosed by JP-2007-334205-A, the heat conductor having an increased diameter faces an inner circumferential surface of the fixing belt and heats the fixing belt. However, since the great heat conductor decreases the curvature of the fixing belt, the recording medium discharged from the fixing nip may be wound around the fixing belt.
On the other hand, in order to decrease frictional resistance between the nip formation pad and the fixing belt sliding thereover, a lubricant may be applied between the nip formation pad and the fixing belt. For example, as disclosed by JP-2006-038990-A, a lubricant guide disposed inside a loop formed by the fixing belt guides the lubricant applied to the inner circumferential surface of the fixing belt to a center of the fixing belt in an axial direction thereof. Accordingly, the lubricant guide moves the lubricant to an interface between the nip formation pad and the fixing belt sliding thereover, thus preventing torque required to drive and rotate the fixing belt from increasing and therefore facilitating conveyance of the recording medium by the fixing belt.
However, since the lubricant guide merely moves the lubricant, the lubricant applied to the inner circumferential surface of the fixing belt may be consumed over time. Moreover, if an additional component to retain the lubricant on the fixing belt is employed, it may upsize the fixing device.